In lithographic printing, ink receptive regions, known as image areas, are generated on a hydrophilic surface. When the surface is moistened with water and ink is applied, the hydrophilic regions retain the water and repel the ink, and the ink receptive regions accept the ink and repel the water. The ink is transferred to the surface of a material upon which the image is to be reproduced. Typically, the ink is first transferred to an intermediate blanket, which in turn transfers the ink to the surface of the material upon which the image is to be reproduced.
Imageable elements useful as lithographic printing plates, also called printing plate precursors, typically comprise an imageable layer applied over the surface of a hydrophilic substrate. The imageable layer includes one or more radiation-sensitive components, which may be dispersed in a suitable binder. Alternatively, the radiation-sensitive component can also be the binder material.
If after exposure to radiation, the exposed regions of the imageable layer are removed in the developing process, revealing the underlying hydrophilic surface of the support, the element is positive working. Conversely, if the developing process removes the unexposed regions, and the exposed regions remain, the element is negative working. In each instance, the regions of the radiation-sensitive layer (i.e., the image areas) that remain are ink-receptive and the regions of the hydrophilic surface revealed by the developing process accept water, typically a fountain solution, and repel ink.
Negative working imageable elements typically comprise a negative working imageable layer on a support. Prior to exposure, the imageable layer is soluble in or penetrable by a developer so that it is removable by the developer. On exposure, the exposed regions of the imageable layer are “hardened,” that is they become insoluble or impenetrable so that they are no longer removable by the developer.
In these single layer elements, the imageable layer must (1) have good imaging characteristics using conventional imaging techniques, and (2) provide a good printing surface, with good resistance to the chemicals used in lithographic printing, after imaging. These two requirements are often incompatible. Modifications of the imageable layer that improve imaging characteristics, i.e., increase imaging speed and/or resolution, often adversely affect the on-press performance of the resulting lithographic printing plate. Modifications of the imageable layer that improve on-press performance often adversely affect photospeed and/or resolution. Thus, a need exists for an imageable element in which both the imaging characteristics of the imageable layer and the performance properties of the resulting printing plate can be independently optimized.